The only real distinction you can make with AH's 3D models is the AH1 and AH2 stuff. The AH1 models were designed for the hardware of the time, with very low poly counts and for low res art (256 by 256 IIRC). They have lots of issues that make them a pain to skin including patched, stretched, distorted or duplicated areas and they also lack things like material files, bump and spec maps.
While the AH2 era skins have improved in poly count over the years and recently for bombers in resolution, they are all fundamentally the same to skin. They all have little or no stretching or duplication of the major external parts and all have bump and spec maps etc.
I don't know but I'd guess HTC will probably work through all the AH1 stuff (the "ancient" list) before redoing any of the earliest AH2 shapes. Once the graphics update is done that will probably be a couple of years work at the present rate, depending on how many new rides are also introduced.